1. Field of the Invention
The present invention relates to a photo mask used in the fabrication of a semiconductor device, and more particularly to a photo mask capable of preventing an occurrence of a notching phenomenon an a photoresist film pattern, caused by an irregular light reflection generated at a depressed slant surface of a layer to be patterned upon a light exposure.
2. Description of the Prior Art
As a semiconductor device has an increased degree of integration, the space between a contact hole adapted to connect upper and lower conducting lines and the wiring surrounding the contact hole is decreased. Moreover, the increased integration degree results in an increase in the aspect ratio, that is the ratio of diameter to depth in the contact hole.
As a conducting layer is coated over the contact hole with the increased aspect ratio, a large depression is formed on the surface of the conducting layer overlapping with the contact hole. When a light exposure process is carried out under a condition that a photoresist film has been coated over the conduction layer, an irregular reflection of light through the photoresist film occurs at the depressed surface portion of the conduction layer. As a result, the photoresist film is exposed to light even at its undesired portion. When the light-exposed photoresist film is subjected to development, a notching phenomenon in which a pattern of the photoresist film is partially removed at its side walls may occur.
FIG. 1 is a plan view of a conventional structure of a semiconductor device involving a notching phenomenon occuring at a photoresist film pattern. As an upper layer is deposited over a semiconductor substrate 1, a depression 3 is formed on the upper layer due to a profile of the structure of the semiconductor substrate disposed beneath the upper layer. When a photoresist film pattern 2 is formed over the upper layer so as to pattern the upper layer, a notching phenomenon occurs at a poxriot of the photoresist film pattern 2 adjacent to the depression 3, in a severe case, the photoresist film pattern 2 is cut, such a notching phenomenon means that the photoresist film is undesirably removed, even at a portion that is intended to remain, due to an undesirable light exposure at the portion of photoresist film caused by an irregular reflection of light through the photoresist film occurring at the depressed surface portion of the upper layer.
Such an occurrence of the notching phenomenon at the photoresist film will be described in detail in conjunction with a method for fabricating a semiconductor device illustrated in FIGS. 2A and 2B, each of which is a cross-sectional view taken along the line II--II of FIG. 1.
In accordance with the illustrated method, first, a lower layer pattern 15 is formed on a semiconductor substrate 11, as shown in FIG. 2A. On the resulting structure, an intermediate layer pattern 16 is formed such that it overlaps with a predetermined portion of the structure, thereby exposing a predetermined portion of the semiconductor substrate 11. An upper layer 17, for example, a metal layer, is then deposited over the resulting structure to a predetermined thickness, over the upper layer 17, a positive photoresist film 18 is coated. Thereafter, the photoresist film 18 is subjected to a light exposure using a photo mask 20 including a quartz substrate formed with a chromium pattern. Since a depression is formed on a surface portion of the upper layer 17 disposed over an area where the lower layer pattern 15 and the intermediate layer pattern 16 are not disposed, light transmitted through the photoresist film during the light exposure is irregularly reflected at a slant surface portion of the upper layer 17 disposed adjacent to the depression, as shown in FIG. 2A. As a result, the photoresist film 18 is exposed even at its portion intended not to be exposed to the light.
Thereafter, a development is carried out to remove the light-exposed portion of the photoresist film 18 and thereby to form a photoresist film pattern 18A, as shown in FIG. 2B. By referring to FIG. 2B, it can be found that an inner portion of the photoresist film pattern 18A has been undesirably removed, that is, a notching phenomenon has occurred.
Subsequently, an upper layer pattern is formed by etching an exposed portion of the upper layer 17. Since the upper layer pattern has the same shape as the photoresist film pattern 18A undesirably notched, the width of the upper layer pattern is smaller than the intended width. As a result, an increase in electrical resistance may occur in a severe case, a disconnection may occur.
In other words, since the upper layer has a severely slanted surface portion at a region where a step is formed due to the lower layer, the non-exposure portion of the photoresist film is undesirably exposed to light reflected by the slant surface of the upper layer upon the light exposure for forming the upper layer pattern. Such a phenomenon occurs more severely where the reflection surface is made of a metal exhibiting a high reflection factor.
FIG. 3 is a plan view of another conventional structure of a semiconductor device involving a notching phenomenon occurring at a photoresist film pattern. In FIG. 3, elements respectively corresponding to those in FIG. 1 are denoted by the same reference numerals. As an upper layer is deposited over a semiconductor substrate 1, a depression 3 is formed on the upper layer due to a profile of the structure of the semiconductor substrate disposed beneath the upper layer. When a photoresist film pattern 2 formed over the upper layer so as to pattern the upper layer, a notching phenomenon occurs at a portion of the photoresist film pattern 2 adjacent to the depression 3. In a severe case, the photoresist film pattern 2 is cut. In other words, the photoresist film is undesirably removed, even at a portion that is intended to remain, during development thereof due to an undesirable light exposure at the portion of photoresist film caused by an irregular reflection of light through the photoresist film occurring at the depressed surface portion of the upper layer, as mentioned above in conjunction with FIG. 1.
Such an occurrence of the notching phenomenon at the photoresist film will be described in detail in conjunction with a method fox fabricating a semiconductor device illustrated in FIGS. 4A and 4B, each of which is a cross-sectional view taken along the line IV--IV of FIG. 3. The structure shown in FIGS. 4A and 4B is substantially similar to that in FIGS. 2A and 2B except for the position of the photoresist film pattern that is shifted due to a shifted position of the photo mask. In FIGS. 4A and 4B, elements respectively corresponding to those in FIGS. 2A and 2B ale denoted by the same reference numerals.
In accordance with this method, first, a lower layer pattern 15 is formed on a semiconductor substrate 11, as shown in FIG. 4A. An intermediate layer pattern 16 is then formed on the resulting structure such that it overlaps with a predetermined portion of the structure, thereby exposing a predetermined portion of the semiconductor substrate 11. Over the resulting structure, an upper layer 17, for example, a metal layer is deposited to a predetermined thickness. A positive photoresist film 18 is then coated over the upper layer 17. Thereafter, the photoresist film 18 is subjected to a light exposure using a photo mask 20 including a quartz substrate formed with a chromium pattern. Since the upper layer 17 has a depression at its surface portion disposed over an area where the lower layer pattern 15 and the intermediate layer pattern 16 are not disposed, the photoresist film 18 is exposed even at its non-exposure portion to light transmitted through the photoresist film during the light exposure and then reflected by a slanted surface portion of the upper layer 17 disposed adjacent to the depression, as shown in FIG. 4A.
Thereafter, the light-exposed portion of the photoresist film 18 is removed by a development, thereby forming a photoresist film pattern 18B, as shown in FIG. 4B. By referring to FIG. 4B, it can be found that a notching phenomenon has occurred at a portion of the photoresist film pattern 18B overlapping with the depression.
Subsequently, an upper layer pattern is formed by etching an exposed portion of the upper layer 17. Since the upper layer pattern has the same shape as the photoresist film pattern 18B undesirably notched, the width of the upper layer pattern is smaller than the intended width. As a result, an increase in electrical resistance may occur. In a severe case, a disconnection may occur.
As apparent from the above description, the conventional structures have a problem of the occurrence of the notching phenomenon in that a portion of the photoresist film pattern is undesirably removed because the photoresist film is undesirably exposed at its non-exposure portion to light reflected by a slanted surface of upper layer, formed due to the step of the lower layer pattern, upon the light exposure for forming the upper layer pattern. Such a problem is more severe where the slanted surface is made of a metal exhibiting a high reflection factor.
In order to solve the above-mentioned problem, there has been proposed a method wherein an upper surface of a substrate is planarized by coating a material exhibiting a high flowability such as boro phosphor silicate glass (BPSG) over a lower layer pattern on the substrate, thereby reducing an irregular reflection of light. There has also been proposed a method of an anti-reflection layer made of a material exhibiting a low reflection factor. In this case, the anti-reflection layer is coated over an upper layer or a photoresist film prior to a formation of a photoresist film pattern. However, these methods require additional steps, as compared to the general photoresist film pattern formation. As a result, they have the problems of a degradation in productivity, an increased fabrication cost and a complex fabrication.